A significant feature of stage lighting systems is the projection of images by stage lighting instruments. Images are typically formed by passing a light beam through a light pattern generator or “gobo” and projecting the image formed thereby. A gobo therefore operates as a light stencil, blocking certain portions of the light beam and passing other portions. A typical configuration for projecting a pattern of light has a gobo placed in a projection gate located at a focal plane of a projection lens and illuminated by a light source. A typical theatrical lighting projector, also called a luminaire comprises a light source, a reflector which focuses light rays to the focal plane, and one or more lenses to project an image of the gobo at the focal plane.
It is desirable to make the projector, and thus the gobos, as small as possible. For this reason, it is often necessary to focus the source's energy into a very small area. This creates intense heat at the focal plane. Typically, heat resistant gobos have been fabricated as a layer of light reflective materials, such as aluminum deposited on a surface of a transparent plate such as heat resistant glass. The light reflective layer has an opening which is in the shape of the image. A portion of the light beam passes through the opening to produce a beam having the shape of the image. The reflective layer serves to reflect a portion of the light beam which does not pass through the opening.
Glass gobos of this type are very resistant to the intense heat present at the projection lens focal plane. Glass gobos of this type are manufactured by a relatively expensive and time-consuming process, e.g., that described in U.S. Pat. No. 4,779,176.
The process described in U.S. Pat. No. 4,779,176 requires a layer of positive photoresist material to be deposited in the shape of a desired image onto a large, thin sheet of transparent glass. A thin layer of aluminum is then deposited over the glass and the photo resist layer. A multi-layer dielectric coating deposited over the aluminum layer forms a “dark mirror”, which is a low reflectivity surface that absorbs visible light. The glass sheet and the various coatings are then exposed to solvents which dissolves the photo resist and lifts all the layers of material immediately over the photo resist while having no effect on the glass. The solvents etch process produces an opening through the deposited layers which is in the shape of the desired image.
This process requires fabricating a photo mask having the desired image formed therein to facilitate deposition of the photo resist layer. Due to the significant lead times required to manufacture the photo mask, and thus the finished gobos, this process is not typically suitable for small, “made-to-order” production runs.
In an effort to overcome the expense and time required to quickly produce glass gobos, prior art methods have employed a laser operating in the near IR to ablate reflective material from a transparent substrate. Such methods are described in U.S. Pat. No. 5,728,994.
As described in U.S. Pat. No. 5,728,994, a light reflective layer is deposited on a surface of a transparent plate. A laser marking system writes an image onto the transparent plate having a reflective layer bonded thereto. The reflective layer is highly reflective to visible light and is absorptive of certain wavelengths of near infra-red radiation, in the range of 850 to 2000 nm. The reflective layer of the blank gobo absorbs the energy of the laser beam and is ablated away from the transparent plate, leaving an opening in the shape of the desired image. The reflective layer can be a four-layer stack of enhanced aluminum, applied by a vacuum-deposition process to produce a coating that is highly reflective of visible light, absorptive of near infra-red radiation at 1.06 micrometers, and stable at high temperatures.
This prior art operated by depositing on a transparent plate a layer of reflective material which reflects visible light and absorbs certain wavelengths of near infra-red radiation; generating a laser beam having a given beam diameter at a certain infra-red wavelength. It may also direct the laser beam onto the transparent plate; steering the laser beam across a surface of the transparent plate; allowing the energy of the beam to ablate reflective material from certain areas of the reflective layer; and switching the laser beam on and off to control which areas of the reflective layer are affected by the laser beam.
Drawbacks of the IR method include:
1. Back reflections
2. Low resolution
3. Requires the use of a high maintenance laser, water cooled, etc.
1.06 micron lasers are large, and require water cooling. Therefore they are expensive to maintain, and the minimum spot size is governed by the wavelength of the laser. Typically, this can be, for example, 0.001″ when written over a 1.0″ diameter field of view. The resulting imagery is coarse.